When an aqueous solution containing divalent iron ions is left over, divalent iron ions are oxidized to be trivalent iron ions and iron (III) hydroxide is precipitated.
The precipitation of iron (III) hydroxide in an alloy plating solution containing divalent iron ions may be suppressed for example by a method in which a compound capable of forming stable complex ions with trivalent iron ions such as a dicarboxylic acid is added (Patent document 1). By adding a dicarboxylic acid such as malonic acid and adjusting the pH to 1.5, trivalent iron ions are stabilized as complex ions and occurrence of the precipitation is suppressed.
However, although the above complexing agent can suppress occurrence of the precipitation, it cannot suppress oxidation of divalent iron ions to trivalent iron ions. As a result, when it is used in a plating solution, a plating film being stable in composition could not be obtained because the quantities of electricity required for deposition of divalent and trivalent ions are different.
It has been also known that an addition of a reducing agent can suppress production of trivalent iron ions. For example, in Patent document 2, a reducing agent such as L-ascorbic acid and gallic acid is added to an iron group alloy plating solution containing divalent iron ions and the pH is adjusted to 1 to 5, in order to suppress production of trivalent iron ions.
However, production of trivalent iron ions could not be sufficiently suppressed even with the reducing agent such as L-ascorbic acid, as mentioned above.